Time for El Niño's half brother to take a bow?

Hurricane Bill as it spun up in August over the Atlantic Ocean. A recently-discovered form of El Niño could become a boon to researchers trying to produce more-accurate seasonal forecasts for the Atlantic hurricane season.

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For those of us living on the US East and Gulf Coasts, the 2009 Atlantic hurricane season has been pretty quiet -- thanks in no small part to El Niño, forecasters note. (Wipe hand across forehead here.)

But over the past few years, researchers have uncovered an odd form of El Niño. Think of it as El Niño's half brother. Now, a team of atmospheric scientists from Korea, Germany, and the US suggests that this form of El Niño may become more common than the El Niños we've experienced up to now.

The likely cause of the shift? Global warming, the team says -- although other researchers caution that this half brother may be yet another form of natural variation that appears and disappears on longer time scales than El Niño does.

The team, whose results appear in the current issue of the journal Nature, posits that global warming's effect on ocean temperatures in the Pacific have changed how those temperatures shift with depth -- prompting the emergence of El Niño's half brother.

For its part, El Niño shows up as a vast pool of unusually warm ocean water that has migrated from the far western Pacific east along the equator until it bumps into South and Central America.

When such a large source of heat to the atmosphere shifts locations so dramatically, atmospheric circulation patterns shift too. So El Niño's arrival can alter seasonal rainfall, drought, and other weather patterns far from where the warm pool has docked. Indeed, a quiet North Atlantic hurricane season represents one of these "teleconnections" during an El Niño year.

El Niños crop up once every 3 to 8 years. During the "off" years, that warm pool of ocean water usually migrates back to the far western Pacific, accompanied by another set of long-range changes to atmospheric circulation patterns. This has become known as La Niña.

Two years ago, however, a team of Japanese scientists published a study in the Journal of Geophysical Research showing that a kind of half El Niño had been lurking undiscovered in climate records for a period that runs from 1979 to 2005.

Traveling half-way

In this case, instead of migrating all the way across the tropical Pacific, the warmest portion of the warm pool hunkers down half-way across, leaving cooler water to the east and west. The Japanese dubbed this El Niño Modoki -- essentially pseudo-El Niño.

The team's curiosity was piqued by weather patterns in 2004 around the Pacific rim. It was supposed to be an El Niño year, but the resulting weather patterns didn't match expectations. Among them: The Atlantic hurricane season was far more active than seasonal forecasts had indicated it should be. As the Japanese scientists took a closer look at historical weather data, they uncovered this pseudo-El Niño.

What kind of effect can it have? Last July, a trio of tropical-cyclone scientists at Georgia Tech in Atlanta published a paper in the journal Science showing that during these half-brother El Niños, North Atlantic hurricane activity reached normal or above-normal levels, rather than the expected below-normal pace.

Better seasonal hurricane forecasts?

That sounds grim if you combine the frequency of these pseudo events with the return rate for La Niña, which also leads to active hurricane seasons. The good news: Apparently these pseudo events are easier to forecast ahead of time than El Niño itself, which holds the promise of improving seasonal hurricane forecasts, says Peter Webster, an atmospheric scientist at Georgia Tech and one of the members of the team reporting the results.

The team received a pat on the back from Chris Landsea, a scientist at NOAA's hurricane forecast hub in Miami. He writes in an email that this work "may provide an advance in the field of seasonal hurricane forecasting."

At the least, he adds, it helps explain why the 2004 hurricane season was far more intense than seasonal forecasts suggested.

Global warming's potential role

The the prospect that global warming will drive an increase in this pseudo El Niño comes from a modeling exercise, in which the team reporting the results first identified the models that did the best job of capturing El Niño activity. Then they ran projections assuming business-as-usual greenhouse gas emissions through the rest of the century. These pseudo El Niños became more common than the traditional El Niños -- as much as five times more common.

Researchers still need to learn more about this phenomenon, note Karumuri Ashok and Toshio Yamagata, two of the Japanese scientists who uncovered this phenomenon in the first place. Not the least of which is testing the notion that this pseudo El Niño is itself a cyclical feature that may appear and disappear on timescales longer than El Niño's -- decades to centuries, rather than every few years.

If all of this seems a little too much, here's comedian Chris Farely's take on El Niño: